Charging circuit for secondary battery

ABSTRACT

A charging circuit for a secondary battery includes a constant-voltage circuit part outputting one of a plurality of predetermined constant voltages and charges the secondary battery by applying the constant voltage thereto, a detection circuit part detecting a battery voltage of the secondary battery, and a control circuit part controlling the selection of the constant-voltage in response to the detected battery voltage. Another charging circuit includes a constant-current circuit part outputting, to the secondary battery, one of two predetermined constant currents, a constant-voltage circuit part charging the secondary battery by applying a predetermined constant voltage thereto, a battery voltage detection circuit part detecting a battery voltage of the secondary battery, a charge current detection circuit part outputting a predetermined charge completion signal, and a charge control circuit part stopping operations of the constant-current circuit part and constant-voltage circuit part when receiving the charge completion signal.

TECHNICAL FIELD

The present invention generally relates to charging circuits forrechargeable secondary batteries, and more particularly to a chargingcircuit for a secondary battery that can quickly charge and avoidgeneration of noise in a frequency band having negative effect onequipment, such as a mobile phone, using the charging circuit.

BACKGROUND ART

As charging methods of a lithium ion battery, when divided roughly, aconstant-current/constant-voltage charge method and a pulse chargemethod are used. In the constant-current/constant-voltage charge method,it is possible to shorten charging time by increasing the chargingcurrent for a lithium ion battery and by making the constant voltageapplied to the lithium ion battery in charging a little higher than thefull charge voltage of the battery. However, when the lithium ionbattery is overcharged, there is a possibility that the performance ofthe battery is degraded. On the other hand, the pulse charge methodcauses little damage to the battery since an idle period is taken duringthe charging of the lithium ion battery.

As such pulse charge methods, there are three methods as follows.

As disclosed in Japanese Laid-Open Patent Application No. 6-113474,there is a first method that completes charging when the voltage in theidle period reaches a predetermined voltage.

There is a second method that makes conditions for starting the chargingand suspending the charging, and repeats the charging and suspension ofthe charging under the conditions. The charging is completed when thecharging suspension period lasts equal to or more than a predeterminedtime, or when the ratio of the charging period to the chargingsuspension period exceeds a predetermined value. For example, thecharging is suspended when the voltage of the battery reaches a firstvoltage and the charging is restarted when the voltage falls to a secondvoltage during the charging.

As disclosed in Japanese Laid-Open Patent Application No. 7-336908,there is a third method that alternately repeats the charging at a highlevel voltage and a low level voltage and completes the charging whenthe charging current at the low level voltage is equal to or less than apredetermined current value.

However, in the above-described first method, there is a problem in thatthe charging time becomes longer compared with theconstant-current/constant-voltage method. In addition, in theabove-described second method, the charging time is shortened to somedegree compared with the constant-current/constant-voltage method.However, since each of the charging period and the charging suspensionperiod varies drastically between the start of the charging and justbefore the end of the charging, the frequency of switching the chargingperiod and the charging suspension period varies over a wide range.Thus, there is a problem in that noise occurs over a wide frequencyband.

Additionally, in the above-described third method, since currentdetection means for detecting the charging current at the low-levelvoltage is required, a current detection element is serially inserted inthe charging circuit. Thus, there is a problem in that electric powerloss occurs. Further, it is necessary to make the value of a currentdetecting resistance large so as to detect when the charging current iszero. Accordingly, there is another problem in that the electric powerloss becomes greater, and at the same time, a complex circuit isrequired.

Further, generally, a secondary battery is used as a power source inmobile radio communication equipment such as a mobile phone. Especially,a lithium ion battery has a high energy density per unit area and perunit mass. Thus, it is possible to make equipment that includes alithium ion battery smaller and lighter. When charging a lithium ionbattery, the constant-voltage charge method that maintains a voltage ofthe battery to be constant, or the constant-current/constant-voltagecharge method that performs constant-voltage charge afterconstant-current charge is employed. In a charging circuit, irrespectiveof the method applied thereto, charging is completed by detecting thatthe charging current is equal to or less than a predetermined fullcharge current during the constant-voltage charge.

In the following, a description will be given of a conventional chargingcircuit for a secondary battery. FIG. 4 is a diagram showing aconventional charging circuit for a secondary battery. In FIG. 4, thecharging circuit includes an AC adapter 110, an adapter detectioncircuit 112 that detects that the AC adapter 110 is connected, a batteryvoltage detection circuit 116 that detects the voltage of a secondarybattery 114 that is to be charged, a constant-voltage circuit 118 thatperforms constant-voltage charge on the secondary battery 114, a chargecurrent detection circuit 122 that detects the charging current flowingto the secondary battery 114, a resistor R1 across which the chargingcurrent causes a voltage drop, a diode D1 that blocks a current fromflowing from the secondary battery 114 to the AC adapter 110, and acharge control circuit 124 that performs drive control of theconstant-voltage circuit 118. The AC adapter 110 is connected to aterminal 130. The constant-voltage circuit 118 includes aconstant-voltage generation circuit 140 that generates a referencevoltage BE1, a control transistor M1, and an operational amplifier A1.In addition, the charge current detection circuit 122 includes aconstant-voltage generation circuit 142 that generates a referencevoltage BE2 and an operational amplifier A2. Further, the adapterdetection circuit 112 includes a constant-voltage generation circuit 144that generates a reference voltage BE3 and an operational amplifier A3.The resistor R1 is connected between the AC adapter 110 and the controltransistor M1. The diode D1 is connected between the control transistorM1 and the secondary battery 114.

In the following, a description will be given of the operation of thischarging circuit. When the AC adapter 110 is connected to the chargingcircuit via the terminal 130, and the voltage of the AC adapter 110 isequal to or more than a predetermined value, the adapter detectioncircuit 112 outputs a predetermined signal Sg1 to the charge controlcircuit 124. In addition, the battery voltage detection circuit 116detects the battery voltage of the secondary battery 114 and outputs abattery voltage signal Sg2. The charge control circuit 124 starts theoperation when the signal Sg1 is input from the adapter detectioncircuit 112, and outputs a predetermined charge control signal Sg5 tothe constant-voltage circuit 118. The constant-voltage circuit 118starts the constant-voltage charge of the secondary battery 114 when thecharge control signal Sg5 is input. While charging, the diode D1prevents a current from flowing back to the AC adapter 110 from thesecondary battery 114 via the control transistor M1 and the resistor R1.The charging current causes a voltage drop across the resistor R1, andthe resulting voltage is applied to the charge current detection circuit122. When the charge current detection circuit 122 detects from theinput voltage that the charging current is lower than a predeterminedvalue, the charge current detection circuit 122 sends a predeterminedcharge completion signal Sg6 to the charge control circuit 124. When thecharge completion signal Sg6 is input to the charge control circuit 124,the charge control circuit 124 outputs the charge control signal Sg5 andstops the operation of the constant-voltage circuit 118.

As mentioned above, in order to detect the charging current, theconventional charging circuit uses the resistor R1. However, at thebeginning of charging, the charging current is high and a sharp voltagedrop results. Thus, heat generation of the resistor R1 becomes veryhigh. In addition, power loss due to the heat generation is also great.In order to reduce such heat generation and waste of power, it isconceivable to make the resistance value of the resistor R1 small.However, by performing the constant-voltage charge, the current whencharge complete is detected is small, and since the voltage drop acrossthe resistor R1 is low, an input offset voltage of the operationalamplifier Al detecting the generated voltage is not negligible. In otherwords, there is a problem in that the accuracy of detecting the chargingcurrent is deteriorated. Further, there is also a problem in that, sinceoperational amplifiers having a small offset voltage are expensive, themanufacturing cost increases when using them

Furthermore, a problem occurs when a large current is supplied to thesecondary battery at the beginning of charging in a case where thesecondary battery is in an over-discharged state. Accordingly, it isimpossible for such a charging circuit to charge the secondary batterythat is in an over-discharged state.

DISCLOSURE OF THE INVENTION

It is a general object of the present invention to provide an improvedand useful charging circuit for a secondary battery in which theabove-mentioned problems are solved.

A more specific object of the present invention is to provide a chargingcircuit for a secondary battery that is a simple circuit, capable ofshortening the charging time, and at the same time, capable of avoidinggeneration of noise in a frequency band adversely affecting equipmentusing the charging circuit.

Another object of the present invention is to provide a charging circuitthat can detect a full charge state of the secondary battery with highaccuracy, small heat generation and small power loss.

Another and more specific object of the present invention is to providea charging circuit that can reduce the manufacturing cost.

Still another object of the present invention is to provide a chargingcircuit that can also charge the secondary battery that is in anover-discharged state.

In order to achieve the above-mentioned objects, according to one aspectof the present invention, there is provided a charging circuit for asecondary battery including: a constant-voltage circuit part thatselects and outputs one constant voltage among a plurality ofpredetermined constant-voltages in response to an input control signaland charges the secondary battery by applying the selected constantvoltage thereto; a detection circuit part that detects a battery voltageof the secondary battery; and a control circuit part that controls theselection of a constant voltage applied from the constant-voltagecircuit part in response to the detected battery voltage from thedetection circuit part, the control circuit part causing theconstant-voltage circuit part to charge the secondary battery byapplying a predetermined first constant voltage thereto when the batteryvoltage of the secondary battery is equal to or less than the firstconstant voltage, and to charge the secondary battery by alternatelyapplying thereto a predetermined second constant voltage and apredetermined third constant voltage that is lower than the secondconstant voltage in a constant cycle when the battery voltage of thesecondary battery exceeds the first constant voltage.

According to the above-mentioned aspect of the present invention, it ispossible to charge the secondary battery with a high current, since theconstant-voltage charge is performed before the pulse charge. Inaddition, even after the pulse charge is started, the charging isperformed by switching from/to a high-level constant-voltage to/from alow-level constant-voltage in a constant cycle (switching cycle). Thus,since the charging current continues, it is possible to shorten thecharging time. At the same time, it is also possible to set theswitching cycle to a frequency that does not adversely affect equipmentusing the charging circuit.

Additionally, according to another aspect of the present invention, thecontrol circuit part detects completion of charging of the secondarybattery and performs a predetermined charge complete operation, when thebattery voltage of the secondary battery exceeds a predetermined chargecomplete voltage, while causing the constant-voltage circuit part toapply the third constant-voltage to the secondary battery.

According to the above-mentioned aspect of the present invention, it ispossible to positively avoid over-charging.

Additionally, according to another aspect of the present invention, thesecond constant-voltage may be equal to the first constant-voltage.

According to the above-mentioned aspect of the present invention, it ispossible to simplify the circuit and to charge the secondary battery soas not to damage the secondary battery.

Additionally, according to another aspect of the present invention, thesecond constant-voltage may be higher than the first constant-voltage.

According to the above-mentioned aspect of the present invention, it ispossible to shorten the charging time without damaging the secondarybattery, by making the high level voltage during the pulse charge alittle higher than the full charge voltage.

Additionally, according to another aspect of the present invention, thecharging circuit may further include a load circuit part that connects aload in parallel with the secondary battery according to the thirdconstant-voltage output from the constant-voltage circuit part.

According to the above-mentioned aspect of the present invention, it ispossible to stabilize the battery voltage of the secondary battery thatis charged with the third constant voltage during the pulse charge.Thus, detection errors of the charge complete voltage can be reduced. Inaddition, the flexibility of the cycle of the pulse charge can beincreased. Accordingly, it is possible to set the cycle to a frequencythat does not give a negative influence to equipment using the chargingcircuit.

According to another aspect of the present invention, theconstant-voltage circuit part may include a constant-voltage generationcircuit that generates and outputs the first constant voltage, thesecond constant voltage and the third constant-voltage; a voltage switchcircuit that, according to the control signal from the control circuitpart, selects and outputs one of the first constant-voltage, the secondconstant voltage and the third constant voltage output from theconstant-voltage generation circuit; a voltage comparator that comparesthe constant voltage output from the voltage switch circuit with thebattery voltage of the secondary battery and outputs a comparison signalaccording to a comparison result; a control transistor that passes acurrent according to the comparison signal from a predetermineddirect-current power source to the secondary battery; and a diode thatblocks a current flowing from the secondary battery to the predetermineddirect-current power source via the control transistor.

According to the above-mentioned aspect of the present invention, it ispossible to charge the secondary battery by switching from theconstant-voltage charge to the pulse charge and with a simple circuitconstruction.

Additionally, according to another aspect of the present invention,there is provided a charging circuit charging a secondary batteryincluding: a constant-current circuit part that is serially connectedbetween an external direct-current power source and the secondarybattery, and outputs, to the secondary battery, one of first and secondconstant currents in response to an input control signal; aconstant-voltage circuit part that is connected in parallel with theconstant-current circuit part, and charges the secondary battery byapplying a predetermined constant voltage thereto; a battery voltagedetection circuit part that detects and outputs a battery voltage of thesecondary battery; a charge current detection circuit part that outputsa predetermined charge completion signal when the constant-voltagecircuit part stops outputting a current; and a charge control circuitpart that stops operations of the constant-current circuit part and theconstant-voltage circuit part when the charge completion signal isinput, wherein, when the battery voltage of the secondary battery islower than a predetermined voltage, the charge control circuit partoutputs, to the constant-current circuit part, the control signal tocause the constant-current circuit part to output the first constantcurrent, and when the battery voltage of the secondary battery is equalto or greater than the predetermined voltage, the charge control circuitpart outputs, to the constant-current circuit part, the control signalto cause the constant-current circuit part to output the second constantcurrent that is greater than the first constant current.

Further, according to another aspect of the present invention, there isprovided a charging circuit charging a secondary battery including: aconstant-voltage circuit part that is connected between an externaldirect-current power source and the secondary battery, and charges thesecondary battery by applying a predetermined constant-voltage thereto;a battery voltage detection circuit part that detects and outputs abattery voltage of the secondary battery; a charge current detectioncircuit part that outputs a predetermined charge completion signal whena current output from said constant-voltage circuit part becomes apredetermined value; and a charge control circuit part that stops anoperation of said constant-voltage circuit part when the predeterminedcharge completion signal is input, said constant-voltage circuit partincluding: a constant-voltage generation circuit that generates andoutputs the predetermined constant voltage; a voltage comparator thatcompares the battery voltage of the secondary battery with thepredetermined constant voltage, and outputs a comparison signalindicating a comparison result; and a control transistor that passes acurrent according to the comparison signal indicating the comparisonresult from the external direct-current power source to the secondarybattery, and said charge current detection circuit part detecting thecomparison signal output from said voltage comparator, and outputs thepredetermined charge completion signal by determining, from the detectedcomparison signal, that the current passed by said control transistor isa predetermined value.

According to the above-mentioned aspects of the present invention, thecharging is completed by detecting, without a resistor, the chargingcurrent output from the constant-voltage circuit. Thus, there is no heatgeneration and no power loss due to the resistor. Accordingly, it ispossible to detect the full charge state of the secondary battery withhigh accuracy.

In addition, according to the above-mentioned aspects of the presentinvention, in a case where the battery voltage of the secondary batteryis lower than the predetermined voltage, it is possible to charge thesecondary battery with a current having an amount suitable for such acase. Thus, it is possible to charge the secondary battery in anover-discharged state. Furthermore, the above-mentioned charging circuitcan be realized while restraining the increase in the size of thecircuit. Therefore, it is possible to reduce the manufacturing cost.

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of the structure of a chargingcircuit for a secondary battery according to a first embodiment of thepresent invention;

FIG. 2 is a timing chart showing an example of the operation of thecharging circuit 1 in FIG. 1;

FIG. 3 is a flow chart for explaining an example of the operation of acharge control circuit 6 in FIG. 1;

FIG. 4 is a diagram showing a conventional charging circuit;

FIG. 5 is a charging circuit according to a second embodiment of thepresent invention;

FIG. 6A is a diagram showing the variation of a voltage of the secondarybattery with charging time in the circuit shown in FIG. 5;

FIG. 6B is a diagram showing the variation of a charging current withcharging time in the circuit shown in FIG. 5;

FIG. 6C is a diagram showing the variation of a gate voltage of a pMOStransistor with charging time in the circuit shown in FIG. 5;

FIG. 7 is a diagram showing an alternative bipolar transistor;

FIG. 8 is a charging circuit according to a third embodiment of thepresent invention;

FIG. 9A is a diagram showing the variation of the battery voltage of thesecondary battery with charging time in the circuit shown in FIG. 8;

FIG. 9B is a diagram showing the variation of the charging current withcharging time in the circuit shown in FIG. 8;

FIG. 9C is a diagram showing the variation of the gate voltage of thepMOS transistor with charging time in the circuit shown in FIG. 8; and

FIG. 10 is another charging circuit according to the third embodiment ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, a detailed description will be given of a first embodiment of thepresent invention, with reference to the drawings.

First Embodiment

FIG. 1 is a diagram showing an example of the structure of a chargingcircuit for a secondary battery in a first embodiment of the presentinvention. It should be noted that FIG. 1 shows the example of thecharging circuit for a lithium ion battery used for a mobile phone.

In FIG. 1, a charging circuit 1 for a secondary battery includes anadapter detection circuit 2 that outputs a predetermined signal when apower supply voltage from an AC adapter 10 that is a direct-currentpower source is equal to or higher than a predetermined value, a batteryvoltage detection circuit 3 that detects and outputs a positive voltage(hereinafter referred to as a “battery voltage”) Vb of a lithium ionbattery 11 that is a secondary battery, and a constant-voltage circuit 4that charges the lithium ion battery 11 at a constant-voltage.

Further, the charging circuit 1 includes a constant-current circuit 5for precharging that precharges the lithium ion battery 11 with apredetermined constant-current, a charge control circuit 6 that, inresponse to a signal from the adapter detection circuit 2 and a detectedvoltage from the battery voltage detection circuit 3, causes theconstant-voltage circuit 4 to perform charging of the pulse chargemethod on the lithium ion battery 11, and causes the constant-currentcircuit 5 to perform the precharging, and a load circuit 7 connected inparallel with the lithium ion battery 11.

In addition, the constant-voltage circuit 4 includes a constant-voltagegeneration circuit 21, a voltage switch circuit 22, an operationalamplifier 23, a control transistor 24, a diode 25 and a gate controlcircuit 26. The constant-voltage generation circuit 21 generates andoutputs three predetermined constant-voltages E1 through E3. The voltageswitch circuit 22 selects one of the constant-voltages E1 through E3from the constant-voltage generation circuit 21 according to a controlsignal from the charge control circuit 6 and outputs the selected one asa reference voltage Vr. The operational amplifier 23 operates as avoltage comparator, and a control transistor 24 that is a PMOStransistor performs supply control of the charging current from the ACadapter 10 on the lithium ion battery 11. The gate control circuit 26performs operation control of the control transistor 24 according to anoutput signal from the operational amplifier 23. Further, the chargecontrol circuit 6 operates as a control circuit. The constant-voltage E1corresponds to a first constant-voltage, the constant-voltage E2corresponds to a second constant-voltage, and the constant-voltage E3corresponds to a third constant-voltage.

The control transistor 24, the diode 25 and the lithium ion battery 11are serially connected between a power terminal 31 and ground so thatthe charging current is supplied to the lithium ion battery 11. Thepower terminal 31 is supplied with power by the AC adapter 10. The diode25 serves to prevent a current from flowing back to the AC adapter 10from the lithium ion battery 11, in a case where the voltage of thepower terminal 31 is lower than the battery voltage Vb of the lithiumion battery 11.

The voltage switch circuit 22 selects one of the constant-voltages E1through E3 according to a voltage switch signal Ss from the chargecontrol circuit 6, and outputs the selected constant-voltage to aninverting input terminal of the operational amplifier 23. The batteryvoltage Vb of the lithium ion battery 11 is applied to a noninvertinginput terminal of the operational amplifier 23. An output terminal ofthe operational amplifier 23 is connected to a gate of the controltransistor 24 via the gate control circuit 26. In addition, the drivingof the operational amplifier 23 is controlled by the control signal fromthe charge control circuit 6.

On the other hand, the load circuit 7 is a series circuit including aresistor 35 and an NMOS transistor 36. The resistor 35 and the NMOStransistor 36 are serially connected between a positive electrode andground. The NMOS transistor 36 operates in accordance with theconstant-voltage selected by the voltage switch circuit 22. When theNMOS transistor 36 turns ON, the resistor 35 serves as a load to thecontrol transistor 24 of the constant-voltage circuit 4. Theconstant-voltages E1 through E3 are in a relationship where thecondition E2≧E1>E3 is satisfied. When the voltage switch circuit 22selects the constant-voltage E3 as the reference voltage Vr in responseto the voltage switch signal Ss, the NMOS transistor 36 is turned ON.When the constant-voltage E1 or E2 is selected as the reference voltageVr, the NMOS transistor 36 is turned OFF and assumes a shut-off state.

FIG. 2 is a timing chart showing an example of the operation of thecharging circuit 1 shown in FIG. 1. A description will be given of anexample of the operation of each part in FIG. 1, by referring to FIG. 2.

First, the charge control circuit 6 is activated when the AC adapter 10supplies power and a predetermined signal is input from the adapterdetection circuit 2. The battery voltage detection circuit 3 detects thebattery voltage Vb of the lithium ion battery 11 and outputs thedetected voltage value to the charge control circuit 6.

In a case where the battery voltage Vb of the lithium ion battery 11 isequal to or less than a predetermined value V1, the charge controlcircuit 6 activates the constant-current circuit 5 so that prechargingof the lithium ion battery 11 with a predetermined precharging currentis started. In addition, at this moment, the charge control circuit 6stops the operation of the operational amplifier 23 so as to avoid thecurrent flowing to the lithium ion battery 11 via the control transistor24.

The above-mentioned predetermined value V1 may be set to approximately2.5 V when the lithium ion battery 11 is a 4.2 V lithium ion battery,for example. This is because a problem may occur when the lithium ionbattery 11 is suddenly charged by a large current in a case where thelithium ion battery 11 is in an over-discharged state. The prechargingof the lithium ion battery 11 is performed such that the chargingcurrent is reduced when starting the charging. A precharging current Ipis a current for the precharging, and is generally set from a few totens of milliamperes, approximately.

When the battery voltage Vb of the lithium ion battery 11 is raised tobe the predetermined value V1, the charge control circuit 6 determinesthat the lithium ion battery 11 is a normal battery, ends theprecharging by the constant-current circuit 5, and outputs the voltageswitch signal Ss so as to switch the charging from the precharging tothe constant-voltage charging by the constant-voltage circuit 4.Further, when precharging, the operation of the constant-voltage circuit4 is stopped, and the diode 25 prevents the current from flowing to theAC adapter 10 from the lithium ion battery 11.

When the precharging ends, the charge control circuit 6 causes thevoltage switch circuit 22 to select the constant-voltage E1 by thevoltage switch signal Ss. The selected constant-voltage E1 is output tothe inverting input terminal of the operational amplifier 23 as thereference voltage Vr. The output voltage of the constant-voltage circuit4 becomes the constant-voltage E1, and charges the lithium ion battery11 with the constant-voltage E1. A charging current Ic when charging thelithium ion battery 11 with the constant-voltage E1 is as shown in FIG.2. A constant-current limited by the current capacity of the AC adapter10 or that of the control transistor 24 is output from theconstant-voltage circuit 4 as the charging current Ic.

When the battery voltage Vb of the lithium ion battery 11 graduallyincreases and reaches the voltage E1 that is the same as the outputvoltage of the constant-voltage circuit 4, the charge control circuit 6performs operation control on the constant-voltage circuit 4 so as tocharge the lithium ion battery 11 by the pulse charge method. Inaddition, the constant-voltage E1 may be set to 4.2 V that is the fullcharge voltage in a case of the lithium ion battery.

The pulse charge method is a method that charges the lithium ion battery11 by repeatedly switching the output voltage of the constant-voltagecircuit 4 from/to the constant-voltage E2 to/from the constant-voltageE3 with a predetermined cycle. When the voltage of the lithium ionbattery 11 reaches the voltage E1, the charge control circuit 6 outputsthe voltage switch signal Ss to the voltage switch circuit 22 so thatthe voltage switch circuit 22 selects the constant-voltage E3, and setsthe output voltage of the constant-voltage circuit 4 to theconstant-voltage E3. The constant-voltage E3 is lower than theconstant-voltage E1. However, the voltage of the constant-voltage E3 isset such that sufficient charging current Ic can be output to thelithium ion battery 11 immediately after the charge method is switchedto the pulse charge method. For example, the constant-voltage E3 may beset from 4.0 V to 4.1 V in a case of the lithium ion battery.

Next, after a predetermined time T1 has elapsed since the charge controlcircuit 6 outputs the voltage switch signal Ss to the voltage switchcircuit 22 so that the voltage switch circuit 22 selects theconstant-voltage E3, the charge control circuit 6 outputs the voltageswitch signal Ss to the voltage switch circuit 22 so that the voltageswitch circuit 22 selects the constant-voltage E2. The voltage switchcircuit 22 selects and outputs the constant-voltage E2 so that theoutput voltage of the constant-voltage circuit 4 becomes theconstant-voltage E2. The constant-voltage E2 may be set to the samevoltage as that of the constant-voltage E1, or to a voltage that isslightly greater than the constant-voltage E1 by approximately 0.1 V,for example. In addition, it should be noted that FIG. 2 shows a casewhere the constant value E2 is greater than the constant-voltage E1, forexample.

In a case where the constant-voltage E2 is set to the same voltage asthat of the constant-voltage E1, there is no possibility that an excessvoltage is applied to the lithium ion battery 11. Thus, there is nodanger of damaging the lithium ion battery 11. Further, since theconstant-voltage E2 is set to the same voltage as that of theconstant-voltage E1, the circuit may be simplified. However, there is adrawback in that the charging time becomes a little longer. In a casewhere the constant-voltage E2 is set a little greater than theconstant-voltage E1, it is possible to shorten the charging time. At thesame time, it is also possible to reduce the likelihood of damaging thelithium ion battery since the pulse charge method is employed.

Next, after the predetermined time T1 has elapsed since the chargecontrol circuit 6 outputs the voltage switch signal Ss to the voltageswitch circuit 22 so that the voltage switch circuit 22 selects theconstant-voltage E2, the charge control circuit 6 outputs the voltageswitch signal Ss to the voltage switch circuit 22 so that the voltageswitch circuit 22 selects the constant-voltage E3 again. The voltageswitch circuit 22 selects and outputs the constant-voltage E3 again sothat the output voltage of the constant-voltage circuit 4 becomes theconstant-voltage E3. In this way, the charge control circuit 6 causesthe constant-voltage circuit 4 to output the constant-voltages E2 and E3alternately in a constant cycle, until the charging of the lithium ionbattery 11 is completed.

As can be seen from FIG. 2, immediately after the charge method isswitched to the pulse charge method, the charging current Ic isapproximately constant since the charging current Ic is a currentlimited by the current capacity of the AC adapter 10 or that of thecontrol transistor 24, whether the output voltage of theconstant-voltage circuit 4 is the constant-voltage E3 or theconstant-voltage E2. However, as the lithium ion battery 11 is charged,the charging current Ic during the charging with the constant-voltage E3is gradually decreased. Further, when the battery voltage Vb of thelithium ion battery 11 becomes equal to or more than theconstant-voltage E3 as the lithium ion battery 11 is charged, thecharging current Ic does not flow when charging with theconstant-voltage E3. Such a method is similar to the general pulsecharge method that repeats charging and suspension of charging. In sucha charging method, it is possible to avoid damage to the lithium ionbattery 11 and to extend the life of the lithium ion battery 11.

When the lithium ion battery 11 is further charged and the batteryvoltage Vb of the lithium ion battery 11 when charging with theconstant-voltage E3 exceeds a predetermined charge complete voltage Ve,the charge control circuit 6 determines that the lithium ion battery 11is completely charged, stops the operation of the operational amplifier23 so as to stop the operation of the constant-voltage circuit 4, andstops the charging operation to the lithium ion battery 11.

The NMOS transistor 36 of the load circuit 7 is turned ON when thevoltage switch circuit 22 selects the constant-voltage E3. When the NMOStransistor 36 is turned ON, the resistor 35 serves as the load to theconstant-voltage circuit 4. Hence, when the output voltage of theconstant-voltage circuit 4 is switched from the constant-voltage E2 tothe constant-voltage E3, it is possible to shorten the time required forthe battery voltage Vb of the lithium ion battery 11 to reach a stablevoltage. Additionally, it is also possible to shorten the time requiredfor the comparison with the charge complete voltage Ve performed by thecharge control circuit 6. Therefore, it is possible to set the time forcharging the lithium ion battery 11 with the constant-voltage E3 short.Thus, it is possible to increase the flexibility of setting the chargingcycle of the pulse charge to a frequency that does not give an influenceon equipment using the charging circuit.

FIG. 3 is a flow chart for explaining an example of the operation of thecharge control circuit 6. A description will be given of the operationflow of the charge control circuit 6, with reference to FIG. 3. Itshould be noted that the process of each step is performed by chargecontrol circuit 6 unless otherwise stated.

In FIG. 3, first, step S1 detects whether or not the voltage of thepower terminal 31 is equal to or more than a predetermined voltage froma signal input by the adapter detection circuit 2. If it is impossibleto detect that the voltage of the power terminal 31 is equal to or morethan the predetermined voltage (NO in step S1), step S1 is repeated. Ifit is detected that the voltage of the power terminal 31 is equal to ormore than the predetermined voltage (YES in step S1), step S2 determineswhether or not the battery voltage Vb of the lithium ion battery 11 thatis detected by the battery voltage detection circuit 3 exceeds thepredetermined value V1.

In step S2, if the battery voltage Vb of the lithium ion battery 11 isequal to or less than the predetermined value V1 (NO in step S2), stepS3 activates the constant-current circuit 5 so as to precharge thelithium ion battery 11, and the process returns to step S2. On the otherhand, in step S2, if the battery voltage Vb of the lithium ion battery11 exceeds the predetermined value V1 (YES in step S2), step S4activates the operational amplifier 23, and at the same time, causes thevoltage switch circuit 22 to select the constant-voltage E1 and toperform the constant-voltage charging with the constant-voltage E1 onthe lithium ion battery 11.

Thereafter, step S5 determines whether or not the battery voltage Vb ofthe lithium ion battery 11 exceeds the constant-voltage E1. If thebattery voltage Vb of the lithium ion battery 11 is equal to or lessthan the constant-voltage E1 (NO in step S5), step S5 is repeated. Onthe other hand, in step S5, if the battery voltage Vb of the lithium ionbattery 11 exceeds the constant-voltage E1 (YES in step S5), step S6causes the voltage switch circuit 22 to select the constant-voltage E3and causes the constant-voltage circuit 4 to charge the lithium ionbattery 11 with the constant-voltage E3.

Next, step S7 determines whether or not the predetermined time T1 haselapsed since the charging with the constant-voltage E3 is started. Ifthe predetermined time T1 has not elapsed (NO in step S7), the chargingwith the constant-voltage E3 is continued until the predetermined timeT1 has elapsed. In addition, in step S7, if the predetermined time T1has elapsed (YES in step S7), the process proceeds to step S8. Step S8determines whether or not the battery voltage Vb is equal to or morethan the predetermined charge complete voltage Ve. If the batteryvoltage Vb is equal to or more than the charge complete voltage Ve (YESin step S8), the charging of the lithium ion battery 11 is completed andthe process ends.

Further, in step S8, if the battery voltage Vb is less than the chargecomplete voltage Ve (NO in step S8), the process proceeds to step S9.Step S9 causes the voltage switch circuit 22 to select theconstant-voltage E2 and causes the constant-voltage circuit 4 to chargethe lithium ion battery 11 with the constant-voltage E2. Next, step S10determines whether or not the predetermined time T1 has elapsed sincethe charging with the constant-voltage E2 is started. If thepredetermined time T1 has not elapsed (NO in step S10), the chargingwith the constant-voltage E2 is continued until the predetermined timeT1 has elapsed. Further, in step S10, if the predetermined time T1 haselapsed (YES in step S10), the process returns to step S6.

As described above, the charging circuit according to the firstembodiment of the present invention precharges the lithium ion battery11 with the precharging current Ip from the constant-current circuit 5when the battery voltage Vb is equal to or less than the predeterminedvalue V1. When the battery voltage Vb exceeds the predetermined valueV1, the charging circuit performs the constant-voltage charging with theconstant-voltage E1 from the constant-voltage circuit 4. When thebattery voltage Vb is equal to the constant-voltage E1, the chargingcircuit performs constant-voltage switching control on the voltageswitch circuit 22 so that the pulse charging is carried out such thatthe constant-voltages E2 and E3 are alternately output from theconstant-voltage circuit 4 in a constant cycle. Accordingly, by adding asimple circuit, when charging the lithium ion battery, it is possible toshorten the charging time and also to prevent noise generation in afrequency band that gives an influence on equipment using the chargingcircuit.

In the following, a description will be given of a second embodiment ofthe present invention, with reference to the drawings.

Second Embodiment

FIG. 5 is a diagram showing a charging circuit according to the secondembodiment of the present invention. In FIG. 5, the charging circuitincludes an AC adapter B10 that supplies a charging current, an adapterdetection circuit 12 detecting that the AC adapter B10 is connected, abattery voltage detection circuit 16 that detects the voltage of asecondary battery 14, a constant-voltage circuit 18 that performs aconstant-voltage charge on the secondary battery 14, a constant-currentcircuit 20 that supplies a constant-current to the secondary battery 14,a gate voltage detection circuit B22 that detects the voltage of acontrol terminal of a control transistor M1, a diode D1 that blocks acurrent flowing from the secondary battery 14 to the AC adapter B10, anda charge control circuit B24 that performs drive control of theconstant-voltage circuit 18 and the constant-current circuit 20. The ACadapter B10 is connected to a terminal 30. The constant-voltage circuit18 includes a constant-voltage generation circuit 40 that generates areference voltage BE1, the control transistor M1, and an operationalamplifier A1. The gate voltage detection circuit B22 includes aconstant-voltage generation circuit 42 that generates a referencevoltage BE2 and an operational amplifier A2. The adapter detectioncircuit 12 includes a constant-voltage generation circuit 44 thatgenerates a reference voltage BE3 and an operational amplifier A3. Inaddition, the diode D1 is connected between the control transistor M1and the secondary battery 14. The diode D1 prevents a current fromflowing back to the AC adapter B10 from the secondary battery 14 via thecontrol transistor M1. Further, in FIG. 5, the control transistor M1 isshown as a p-channel metal-oxide semiconductor field-effect transistor(hereinafter referred to as a “PMOS transistor”).

In the following, a description will be given of the operation of thecharging circuit according to the second embodiment. When the AC adapterB10 that is a power source of the charging circuit is connected to thecharging circuit via the terminal 30, and the voltage of an inputterminal of the operational amplifier A3 connected to the terminal 30 isequal to or more than the predetermined reference voltage BE3, theadapter detection circuit 12 sends a predetermined signal Sg1 to thecharge control circuit B24. Additionally, the battery voltage detectioncircuit 16 detects the battery voltage of the secondary battery 14,generates a battery voltage signal Sg2, and outputs the signal to thecharge control circuit B24. The charge control circuit B24 is activatedwhen the signal Sg1 is input thereto. The charge control circuit B24outputs a constant-current control signal Sg3 to the constant-currentcircuit 20 when the battery voltage signal Sg2 is input thereto. Theconstant-current circuit 20 is activated when the constant-currentcontrol signal Sg3 is input thereto. The constant-current circuit 20includes two power sources inside and can output one of two kinds ofcurrents in a direction indicated by I_(B) in FIG. 5. The charge controlcircuit B24 outputs a constant-current value switch signal Sg4 with theconstant-current control signal Sg3 to the constant-current circuit 20,when the charge control circuit B24 detects that, from the batteryvoltage signal Sg2 that is input, the battery voltage of the secondarybattery 14 is lower than a predetermined voltage BV1. This is forreducing the charge current, since a problem occurs when the secondarybattery 14 is suddenly charged by a great current in a case where thebattery voltage of the secondary battery 14 is lower than BV1, that is,the secondary battery 14 is in an over-discharged state. Hence, theconstant-current circuit 20 outputs a current having a current valueBI1, when the constant-current value switch signal Sg4 is input to theconstant-current circuit 20. In a case of the lithium ion battery, thevoltage BV1 is set to approximately 2.5 V, and generally, the currentvalue BI1 ranges from a few to tens of milliamperes. As described above,charging of the secondary battery 14 is started when theconstant-current control signal Sg3 is output to the constant currentcircuit 20.

The charge control circuit B24 determines that the secondary battery 14is a normal battery and outputs the constant-current value switch signalSg4 to the constant-current circuit 20, when the secondary battery 14 ischarged by the current having the current value BI1, and the chargecontrol circuit B24 detects that the battery voltage of the secondarybattery 14 reaches the predetermined voltage BV1, according to thebattery voltage signal Sg2 supplied from the battery voltage detectioncircuit 16. Hence, the constant-current circuit 20 outputs a currentvalue BI2 that is larger than the current value BI1 to the secondarybattery 14. The current value BI2 is equal to the full charge currentthat flows to the secondary battery 14 when the constant-voltage chargeis completed. Further, the charge control circuit B24 outputs a chargecontrol signal Sg5 to the constant-voltage circuit 18 so as to activatethe constant-voltage circuit 18. The constant-voltage circuit 18 outputsa charge current to the secondary battery 14 in a direction indicated byBI_(c) in FIG. 5. Subsequently, the secondary battery 14 is charged bythe current output by both the constant-voltage circuit 18 and theconstant-current circuit 20.

Thereafter, when the battery voltage of the secondary battery 14 isfurther raised and reaches a voltage BV2 that is approximately equal tothe reference voltage BE1 of the constant-voltage circuit 18, thebattery voltage of the secondary battery 14 is not raised anymore,maintained to be constant, and only the charging current decreasesgradually. At this moment, the operational amplifier A1 is comparing thebattery voltage of the secondary battery 14 with the reference voltageBE1, and is applying a positive gate voltage (control voltage) to a gate(control terminal) of the pMOS transistor M1 according to thedifference. The higher the battery voltage of the secondary battery 14is, the higher the applied gate voltage becomes. Thus, a drain currentis gradually limited. That is, the charging current supplied to thesecondary battery 14 is gradually decreased. In a case of the lithiumion battery, the voltage BV2 is set to approximately 4.2 V. When thevoltage is further raised, a problem occurs since metallic lithium isseparated inside the secondary battery 14. Even in a conventionalconstant-current constant-voltage charge circuit, when the batteryvoltage of the secondary battery 14 reaches the voltage BV2, theconstant-current charge is switched to the constant-voltage charge.Further, ideally, total charging current starts to be decreasedsimultaneously when the battery voltage of the secondary battery 14reaches the voltage BV2. However, there is some time differencedepending on the progress of chemical reaction inside the battery.

FIGS. 6A, 6B and 6C are graphs diagrammatically showing theabove-mentioned operation. FIG. 6A is a graph showing the variation ofthe battery voltage of the secondary battery 14 with charging time. FIG.6B is a graph showing the variation of the charging current with thecharging time. In addition, FIG. 6C is a graph showing the variation ofthe gate voltage of the pMOS transistor M1 with the charging time. FIG.6B shows each variation of the current A (indicated by a bold line)output by the constant-current circuit 20, the charging current B outputby the constant-voltage circuit 18, and a total charging current Cobtained by adding the current output by the constant-current circuit 20to the current output by the constant-voltage circuit 18. Referring toFIGS. 6A and 6B, the secondary battery 14 is charged by the currenthaving the current value BI1 output from the constant-current circuit 20until the voltage reaches BV1 (until charging time t1). When the batteryvoltage of the secondary battery 14 reaches BV1, the constant-currentcircuit 20 outputs the charging current having the current value BI2,and the constant-voltage circuit 18 also starts to output a chargingcurrent. The charging current output from the constant-voltage circuit18 is a current that is, at first, limited by the current capacity ofthe AC adapter B10 or the current capacity of the pMOS transistor M1,whichever is smaller. FIG. 6B shows the charging current in a case wherethe current capacity of the AC adapter B10 is smaller, for example. Thesecondary battery 14 is charged by the currents output by both theconstant-voltage circuit 18 and the constant-current circuit 20, andthus the battery voltage of the secondary battery 14 is raised andreaches the predetermined voltage BV2.

When a certain amount of time has elapsed after the battery voltage ofthe secondary battery 14 reaches the predetermined voltage BV2, the gatevoltage of the pMOS transistor M1 starts to be increased gradually, andin response this increase, the current output from the constant-voltagecircuit 18 starts to be decreased gradually. Then, as shown in FIG. 6C,at a charging time t2, the gate voltage of the pMOS transistor M1 israised close to the AC adapter voltage. At this moment, the PMOStransistor of the constant-voltage circuit 18 is cut off, and thecharging current output from the constant-voltage circuit 18 is stopped.In other words, the total charging current is only the current havingthe current value BI2 output from the constant-current circuit 20.

In the charging circuit according to this embodiment, since the currentvalue BI2 is set equal to the value of the full charging current, it ispossible to consider that the charging is completed when the pMOStransistor M1 of the constant-voltage circuit 18 is cut off, and onlythe current having the current value BI2 output from theconstant-current circuit 20 flows to the secondary battery 14.

Accordingly, if the reference voltage BE2 of the gate voltage detectioncircuit B22 is set such that a voltage that is dropped to the lowervalue from the voltage of the AC adapter B 10 by the reference voltageBE2 is equal to the gate voltage at which the pMOS transistor M1 is cutoff, the gate voltage detection circuit B22 outputs a charge completionsignal Sg6 to the charge control circuit B24, when the controltransistor M1 is cut off, that is, the gate voltage of the PMOStransistor M1 input to one of the input terminals of the operationalamplifier A2 is equal to the voltage that is dropped from the voltage ofthe AC adapter B10 by the reference voltage BE2. As described above, thegate voltage detection circuit B22 detects that a predetermined currentflows to the secondary battery 14, by detecting the gate voltage of thePMOS transistor M1. Thus, the gate voltage detection circuit B22 can becalled as a charging current detection circuit. When the chargecompletion signal Sg6 is input to the charge control circuit B24, thecharge control circuit B24 outputs the charge control signal Sg5 and theconstant-current control signal Sg3 to the constant-voltage circuit 18and the constant-current circuit 20, respectively, and stops theoperations of both circuits.

In the charging circuit according to this embodiment, a resistor fordetecting the charge current is not required. Thus, there is no heatgeneration or power loss due to the resistor. Accordingly, it ispossible to detect the full charge state with high accuracy. Further, itis possible to select the current value of the current output from theconstant-current circuit 20 from among different current values.Therefore, it is possible to charge even an over-discharged battery andthe like without adding a new circuit.

In addition, in the charging circuit according to this embodiment, thegate voltage detection circuit B22 sets the voltage that is dropped fromthe voltage of the AC adapter B10 by the reference voltage BE2 equal tothe gate voltage at which the pMOS transistor M1 is cut off, by usingthe constant-voltage generation circuit 42 that generates the referencevoltage BE2. However, this is the same thing as to set the chargecomplete voltage equal to the gate voltage at which the pMOS transistorM1 is cut off, by using the constant-voltage generation circuit 42 thatgenerates the charge complete voltage.

Furthermore, it should be noted that a pMOS transistor is used for thecontrol transistor M1 in FIG. 5, however, a similar effect can beobtained even when a bipolar PNP transistor as shown in FIG. 7 is usedinstead. In this case, the reference voltage BE2 of the gate voltagedetection circuit B22 may be set such that the voltage dropped from thevoltage of the AC adapter B10 by the reference voltage BE2 is equal to abase voltage at which the bipolar PNP transistor is cut off.

Third Embodiment

FIG. 8 is a diagram showing a charging circuit for the secondary battery14 according to a third embodiment of the present invention. In FIG. 8,those parts that are the same as those corresponding parts in FIG. 5 aredesignated by the same reference numerals, and a description thereofwill be omitted. The charging circuit according the third embodimentfurther includes, in addition to the charging circuit shown in FIG. 5, acharge current control circuit 50 that controls a charging currentoutput from the pMOS transistor M1, and a load resistor R2.Additionally, a diode D3 is connected between the operational amplifierAl of the constant-voltage circuit 18 and the PMOS transistor M1. Thecharge current control circuit 50 includes a constant-voltage generationcircuit 46, an operational amplifier A4 and a diode D2. One terminal ofthe load resistor R2 is connected to ground, and the other terminal isconnected to a gate terminal of the pMOS transistor M1.

FIGS. 9A, 9B and 9C show the variation of the battery voltage of thesecondary battery 14 with the charging time, the variation of thecharging current, and the variation of the gate voltage of the pMOStransistor M1, respectively. FIG. 9B shows the current A (indicated by abold line) output from the constant-current circuit 20, the chargingcurrent B output from the constant-voltage circuit 18, and the totalcharging current C that is obtained by adding the current output fromthe constant-current circuit 20 to the current output from theconstant-voltage circuit 18. The charging circuit according to the thirdembodiment operates similarly to the charging circuit according to thesecond embodiment, until the battery voltage of the secondary battery 14reaches the predetermined voltage BV1 (until the charging time becomest1). The charge control circuit B24 outputs the constant-current valueswitch signal Sg4 to the constant-current circuit 20, when detectingthat the battery voltage of the secondary battery 14 reaches thepredetermined voltage BV1 from the battery voltage signal Sg2 outputfrom the battery voltage detection circuit 16. Hence, theconstant-current circuit 20 outputs the current value BI2 that is largerthan the current value BI1 to the secondary battery 14. Further, thecharge control circuit B24 outputs the charge control signal Sg5 to theconstant-voltage circuit 18 and the charge current control circuit 50 soas to activate the constant-voltage circuit 18 and the charge currentcontrol circuit 50, respectively.

At first, since the battery voltage of the secondary battery 14 is stilllow, the output of the operational amplifier Al of the constant-voltagecircuit 18 is approximately 0 V. On the other hand, the operationalamplifier A4 of the charge current control circuit 50 compares the gatevoltage of the pMOS transistor M1 with the voltage dropped from thevoltage of the AC adapter B10 (the voltage of the terminal 30) by thereference voltage BE4, and outputs the voltage so that the gate voltageof the pMOS transistor M1 is maintained to be constant and equal to thevoltage dropped from the voltage of the AC adapter B10 by the referencevoltage BE4. At this moment, the diode D3 of the constant-voltagecircuit 18 blocks the current flowing from the gate terminal of the pMOStransistor M1 to the operational amplifier Al. After all, the gatevoltage of the pMOS transistor M1 is maintained to be constant, and thedrain current of the pMOS transistor M1, that is, the charging currentoutput from the constant-voltage circuit 18 is constant at the currentvalue BI3.

However, due to the performance of the pMOS transistor M1, there is acase where the predetermined drain current does not flow even when thepredetermined gate voltage is applied. Thus, as shown in FIG. 8, byarranging the load resistor R2, fine adjustment of the gate voltage isperformed so that the predetermined drain current flows. As describedabove, the secondary battery 14 is charged by both of the constantcurrent having the current value BI2 and the drain current having thecurrent value BI3.

When the battery voltage of the secondary battery 14 is increased andreaches the predetermined voltage BV2, the output voltage of theoperational amplifier Al of the constant-voltage circuit 18 isincreased, and the current starts to flow from the operational amplifierAl to the gate terminal of the pMOS transistor M1 via the diode D3.Therefore, the gate voltage of the pMOS transistor M1 is increased.Instead, the output of the operational amplifier A4 of theconstant-current control circuit 50 falls to approximately 0 V. Thus,the current stops to flow from the operational amplifier A4 to the gatevoltage of the pMOS transistor M1 via the diode D2. When the gatevoltage of the pMOS transistor M1 is increased, the drain current outputfrom the pMOS transistor M1 decreases. As the secondary battery 14 isfurther charged, the gate voltage of the pMOS transistor M1 is furtherincreased, and the pMOS transistor M1 is cut off. At this moment, thetotal current flowing to the secondary battery 14 has the current valueBI2 that is equal to the full charging current flowing to the secondarybattery 14 when the constant-voltage charge is completed.

When the reference voltage BE2 of the gate voltage detection circuit B22is set such that the voltage dropped from the voltage of the AC adapterB10 by the reference voltage BE2 is equal to the gate voltage at whichthe pMOS transistor M1 is cut off, the gate voltage detection circuitB22 outputs the charge completion signal Sg6 to the charge controlcircuit B24 when the control transistor M1 is cut off. When the chargecontrol signal Sg6 is input to the charge control circuit B24, thecharge control circuit B24 outputs the charge control signal Sg5 and theconstant-current control signal Sg3 to the constant-voltage circuit 18and the constant-current circuit 20, respectively, and stops theoperations of both circuits.

In the charging circuit according to this embodiment, even immediatelyafter the constant-voltage circuit 18 is driven, it is possible to applythe predetermined gate voltage to the pMOS transistor M1. Accordingly,it is possible to supply the secondary battery 14 with the predeterminedconstant current that is not dependent on the current capacity of the ACadapter B10 or the current capacity of the pMOS transistor M1. Hence,even immediately after the constant-voltage circuit 18 is driven, it ispossible to supply the secondary battery 14 with the charging currenthaving a suitable current value that does not damage the secondarybattery 14.

Additionally, in the charging circuit according to this embodiment, aresistor for detecting the charging current is not required. Thus, thereis no heat generation or power loss due to the resistor. Accordingly, itis possible to detect the full charge state of the secondary batterywith high accuracy. In addition, it is possible to select the currentvalue of the current output from the constant-current circuit 20 fromamong different current values. Therefore, it is possible to charge evenan over-discharged battery and the like without adding a new circuit.

Further, in the charging circuit according to the third embodiment, byusing the constant-voltage generation circuit 42 that generates thereference voltage BE2, the gate voltage detection circuit B22 sets thevoltage dropped from the voltage of the AC adapter B10 by the referencevoltage BE2 equal to the gate voltage at which the pMOS transistor M1 iscut off. However, this is the same thing as to set the charge completevoltage equal to the gate voltage at which the pMOS transistor M1 is cutoff, by using the constant-voltage generation circuit that generates thecharge complete voltage. In addition, by using the constant-voltagegeneration circuit 46 that generates the reference voltage BE4, thecharge current control circuit 50 sets the voltage dropped from thevoltage of the AC adapter B10 by the reference voltage BE4 equal to thegate voltage of the PMOS transistor M1 that outputs the predeterminedconstant current. However, this is the same thing as to set, by usingthe constant-voltage generation circuit that generates a certain controlvoltage, the control voltage equal to the gate voltage of the pMOStransistor M1 that outputs the predetermined constant current.

In addition, in FIG. 8, the control transistor M1 is a pMOS transistor.However, similar effect can be obtained also by the bipolar PNPtransistor as shown in FIG. 7. In this case, the reference voltage BE2of the gate voltage detection circuit B22 may be set such that voltagedropped from the voltage of the AC adapter B10 by the reference voltageBE2 is equal to the base voltage of the bipolar PNP transistor at whichthe bipolar PNP transistor is cut off. Additionally, the referencevoltage BE4 of the constant-voltage generation circuit 46 may be setsuch that a voltage dropped from the voltage of the AC adapter B10 bythe reference voltage BE4 is equal to the base voltage of the bipolarPNP transistor that outputs a predetermined constant current.

Further, in the charging circuit according to this embodiment, thecharge current control circuit 50 maintains the gate voltage of the pMOStransistor M1 to be constant, and passes the predetermined constantcurrent to the secondary battery 14 via the PMOS transistor M1. However,another construction may be used as long as it is possible to pass thepredetermined constant current to the secondary battery 14 via the pMOStransistor M1. A similar effect can be obtained even in such a case.However, when the load resistor R2 is arranged, as shown in FIGS. 8 and10, it is easy to fine adjust the value of the gate voltage applied tothe pMOS transistor M1. For example, even in a case where the PMOStransistor M1 is replaced with another pMOS transistor M1 of a differentmanufacturer, it is possible to simply adjust the gate voltage accordingto the performance of the pMOS transistor M1. Hence, it is possible topass the predetermined constant current to the secondary battery 14,irrespective of the performance of the pMOS transistor M1.

Additionally, in the charging circuit in FIG. 8, the constant currentcircuit 20 may be a constant current circuit that outputs only thecurrent value BI1. FIG. 10 is a diagram showing a charging circuit insuch a case. The constant current circuit 20 has a single current sourcethat outputs a current having a current value BI1, and is controlled bythe constant-current control signal Sg3 that is output from the chargecontrol circuit B24.

In a case where the battery voltage of the secondary battery 14 is lowerthan the predetermined voltage BV1, the constant-current circuit 20 isactivated since the constant-current control signal Sg3 is input by thecharge control circuit B24, and the secondary battery 14 is charged onlywith the current having the current value BI1. When the charge controlcircuit B24 detects, from the battery voltage signal Sg2 output by thebattery voltage detection circuit 16, that the battery voltage of thesecondary battery 14 reaches the predetermined voltage BV1, the chargecontrol circuit B24 sends the constant-current control signal Sg3 to theconstant-current circuit 20 so as to stop the operation of theconstant-current circuit 20. Further, the charge control circuit B24outputs the charge control signal Sg5 so as to activate theconstant-voltage circuit 18 and the charge current control circuit 50.The operations of the charge current control circuit 50 and theconstant-voltage circuit 18 are the same as the operations of thosecorresponding parts in FIG. 8.

In the charging circuit in FIG. 10, a reference voltage BE5 of the gatevoltage detection circuit B22 is different from the reference voltageBE2 of the charging circuits in FIGS. 4 and 8. This reference voltageBE5 is set such that a voltage dropped from the voltage of the ACadapter B10 by the reference voltage BE5 is the same as the voltageapplied to the gate terminal of the pMOS transistor M1 so that the draincurrent of the pMOS transistor M1 is equal to the current value 12.Hence, the charging is changed from the constant-current charge to theconstant-voltage charge. When the gate voltage of the pMOS transistor M1is increased and reaches the voltage that is decreased from the voltageof the AC adapter B10 by the reference voltage BE5, the gate voltagedetection circuit B22 outputs the charge completion signal Sg6 to thecharge control circuit B24. When the charge completion signal Sg6 isinput, the charge control circuit B24 outputs the charge control signalSg5 to the constant-voltage circuit 18 and the constant-current controlcircuit 50 so as to stop their operations.

In the charging circuit as shown in FIG. 10, the constant-currentcircuit 20 may include a single current source. Thus, the size of thecircuit is reduced. As a result, the manufacturing cost is reduced.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese priority applications No.2001-279823 filed on Sep. 14, 2001 and No. 2001-287039 filed on Sep. 20,2001, the entire contents of which are hereby incorporated by reference.

1. A charging circuit for charging a secondary battery, comprising; a constant-current circuit that outputs, to the secondary battery, one of first and second constant currents in response to an input control signal; a constant-voltage circuit that charges the secondary battery by applying a predetermined constant voltage thereto; a battery voltage detection circuit that detects and outputs a battery voltage of the secondary battery; a charge current detection circuit that outputs a predetermined charge completion signal when a predetermined current flows to said secondary battery; and a charge control circuit that stops operations of said constant-current circuit and said constant-voltage circuit when the charge completion signal is input, wherein, when the battery voltage of the secondary battery is lower than a predetermined voltage, said charge control circuit outputs, to said constant-current circuit, the control signal to cause said constant-current circuit to output the first constant current, and when the battery voltage of the secondary battery is equal to or greater than the predetermined voltage, said charge control circuit outputs, to said constant-current circuit, the control signal to cause the constant-current circuit to output the second constant current that is greater than the first constant current.
 2. The charging circuit as claimed in claim 1, further comprising: a direct-current power source that supplies a charging current to the secondary battery.
 3. The charging circuit as claimed in claim 1, wherein the constant-voltage circuit comprises: a constant-voltage generation circuit that generates and outputs the predetermined constant voltage; a voltage comparator that compares the battery voltage of the secondary battery with the predetermined constant voltage, and outputs a comparison signal indicating a comparison result; and a control transistor that passes a current according to the comparison signal indicating the comparison result from the external direct-current power source to the secondary battery.
 4. The charging circuit as claimed in claim 3, wherein the constant-voltage circuit further comprises a diode that blocks a current flowing to the voltage comparator.
 5. The charging circuit as claimed in claim 1, wherein the constant-voltage circuit further comprises a diode that blocks a current flowing from the secondary battery to the external direct-current power source via the control transistor.
 6. The charging circuit as claimed in claim 3, wherein the control transistor is a p channel metal oxide semiconductor field-effect transistor.
 7. The charging circuit as claimed in claim 3, wherein the control transistor is a PNP bipolar transistor.
 8. A charging circuit for charging a secondary battery, comprising: a constant-current circuit that outputs, to the secondary battery, one of first and second constant currents in response to an input control signal; a constant-voltage circuit that charges the secondary battery by applying a predetermined constant voltage thereto; a battery voltage detection circuit that detects and outputs a battery voltage of the secondary battery; a circuit that outputs a predetermined charge completion signal; and a charge control circuit that stops operations of said constant-current circuit and said constant-voltage circuit when the charge completion signal is input, wherein, when the battery voltage of the secondary battery is lower than a predetermined voltage, said charge control circuit outputs, to said constant-current circuit, the control signal to cause said constant-current circuit to output the first constant current, and when the battery voltage of the secondary battery is equal to or greater than the predetermined voltage, said charge control circuit outputs, to said constant-current circuit, the control signal to cause the constant-current circuit to output the second constant current that is greater than the first constant current.
 9. The charging circuit as claimed in claim 8, further comprising; a direct-current power source supplies that a charging current to the secondary battery.
 10. The charging circuit as claimed in claim 8, wherein the constant-voltage circuit comprises; a constant-voltage generation circuit that generates and outputs the predetermined constant voltage; a voltage comparator that compares the battery voltage of the secondary battery with the predetermined constant voltage, and outputs a comparison signal indicating a comparison result; and a control transistor that passes a current according to the comparison signal indicating the comparison result from the external direct-current power source to the secondary battery.
 11. The charging circuit as claimed in claim 10, wherein the constant-voltage circuit further comprises a diode that blocks a current flowing to the voltage comparator.
 12. The charging circuit as claimed in claim 8, wherein the constant-voltage circuit further comprises a diode that blocks a current flowing from the secondary battery to the external direct-current power source via the control transistor.
 13. The charging circuit as claimed in claim 10, wherein the control transistor is a p channel metal oxide semiconductor field-effect transistor.
 14. The charging circuit as claimed in claim 10, wherein the control transistor is a PNP bipolar transistor.
 15. A charging circuit for charging a secondary battery using a constant-current/constant voltage charging method, comprising: a constant-voltage circuit configured to charge the secondary battery in a constant-voltage mode; a constant-current circuit configured to charge the secondary battery in a constant current mode, wherein, when a battery voltage of the secondary battery is lower than a predetermined voltage, the constant-current circuit outputs a first constant current, and when the battery voltage of the secondary battery is equal to or greater than a predetermined voltage, the constant-current circuit outputs a second constant current that is greater than the first constant current.
 16. A charging circuit for charging a secondary battery, comprising; a constant-current circuit that outputs, to the secondary battery, one of first and second constant currents in response to an input control signal; a constant-voltage circuit that charges the secondary battery by applying a predetermined constant voltage thereto; a battery voltage detection circuit that detects a battery voltage of the secondary battery, and outputs a signal representative of a battery voltage of the secondary battery; a charge current detection circuit that outputs a predetermined charge completion signal when a predetermined current flows to said secondary battery; and a charge control circuit that stops operations of said constant-current circuit and said constant-voltage circuit when the charge completion signal is input, wherein, when the battery voltage of the secondary battery is lower than a predetermined voltage, said charge control circuit outputs, to said constant-current circuit, the control signal to cause said constant-current circuit to output the first constant current, and when the battery voltage of the secondary battery is equal to or greater than the predetermined voltage, said charge control circuit outputs, to said constant-current circuit, the control signal to cause the constant-current circuit to output the second constant current that is greater than the first constant current.
 17. The charging circuit as claimed in claim 16, wherein the constant-voltage circuit comprises: a constant-voltage generation circuit that generates and outputs the predetermined constant voltage; a voltage comparator that compares the signal representative of the battery voltage of the secondary battery with the predetermined constant voltage, and outputs a comparison signal indicating a comparison result; and a control transistor that passes a current according to the comparison signal indicating the comparison result from the external direct-current power source to the secondary battery.
 18. A charging circuit for charging a secondary battery, comprising: a constant-current circuit that outputs, to the secondary battery, one of first and second constant currents in response to an input control signal; a constant-voltage circuit that charges the secondary battery by applying a predetermined constant voltage thereto; a battery voltage detection circuit that detects a battery voltage of the secondary battery, and outputs a signal representative of a battery voltage of the secondary battery; a charge completion circuit that outputs a predetermined charge completion signal; and a charge control circuit that stops operations of said constant-current circuit and said constant-voltage circuit when the charge completion signal is input, wherein, when the battery voltage of the secondary battery is lower than a predetermined voltage, said charge control circuit outputs, to said constant-current circuit, the control signal to cause said constant-current circuit to output the first constant current, and when the battery voltage of the secondary battery is equal to or greater than the predetermined voltage, said charge control circuit outputs, to said constant-current circuit, the control signal to cause the constant-current circuit to output the second constant current that is greater than the first constant current.
 19. The charging circuit as claimed in claim 18, wherein the constant-voltage circuit comprises; a constant-voltage generation circuit that generates and outputs the predetermined constant voltage; a voltage comparator that compares the signal representative of the battery voltage of the secondary battery with the predetermined constant voltage, and outputs a comparison signal indicating a comparison result; and a control transistor that passes a current according to the comparison signal indicating the comparison result from the external direct-current power source to the secondary battery. 